How strong is Bitcoin against 51% attack !

[butka]

Cost of harwares:
Hardware = Current hashrate * Price of Antminer S9 / Hash produced by Antminer (per seconds)
Hardware = 36349358,259 * 2400$ / 14
Hardware ~=  6,231,318,559 $

This is one time investment.

So attacker have to pay 3,855,627 $ of electricity per day !

And this is what the attacker would have to pay every day if they wanted to keep the attack alive.

With so much money involved I guess this could be pulled off only by the government (if we don't consider the most important miners currently mining bitcoin). Which would be hard to maintain long term, as the government presumably wouldn't be making any profit out of it. Also knowing that if they somehow managed to destroy bitcoin, there would be numerous other altcoins waiting to take its place, so it wouldn't be worth even contemplating.

[d5000]

I have made similar calculations about a year ago and came to the conclusion that Bitcoin could be 51% attacked with about 2-4% of the value of the "market cap" (with some variance because price/difficulty relation is not always the same). I based my calculations on rented hardware (cloud mining) but essentially the cost seems to be similar.

So I agree: Bitcoin is strong.

But Kakmakr's answer reminded me of a scenario I never was able to get out of my mind, where I guess it could be possible for an attacker to obtain large profits with his attack:

If a double spend were done, investors in Bitcoin will question the security of this technology and the validity of the transactions. [...] A massive dump will follow and the price will take a nose dive. An attack of this magnitude will cause panic and the attackers will gain very little in the long run, because the price will keep going down. The cost to sustain such an attack, also outweigh the gains of doing this.  

What if the attacker just wanted that -  to crash the price - for example, because he's massively shorting coins and would profit of a crash?

He would have to create a double spend to prove that he attacked the chain, but he can do that with his own addresses. So he would not even face charges of theft. (He could, however, face the charge of "damage to property" as he's interfering in the functioning of the Bitcoin system, or something similar.)

This attack scenario, however, depends massively on the amount of coins which can be shorted. Unfortunately, the new future markets would just allow these kinds of massive short operations.

Anybody able to "rebutt" that scenario?

[spartacusrex]

When accepting payments below $1 million dollars.. no one is going to 'Attack-The-Chain'. As you say, too expensive.

When accepting payments over $1 million, just wait a little longer..

If you did have 51% hash power, outrunning a 1 day chain, takes.. Yep.. 50 Days. No ones going to do that. (You only get a 1% boost - as the normal chain grows as per usual, but slower..)

I think to properly attack the network, what you need to do is remove 90% of the hashrate. But probably only a government could do that.. or BitMain (root kit) LOL..  China could just shut all the Chinese miners down. If you could do that, BTC would be in trouble. The blocks would take f-o-r-e-v-e-r-.. the retarget period would take 100's of days. You could hard fork your way to a simpler difficulty, but that's the only way. (But then I suppose they could turn the machines that go PING back on.. and attack properly.. with 90%..)

Anyway, I'd like to see a system introduced that could handle that sort of hash rate loss. Simply allowing the difficulty to trail off after 2 hrs.. has security issues..

It almost happened when BCH attacked the network.

..

The only people who could even think of doing this are those we pay to protect us.  Always the way (when you pay a third party to protect you).